Passive ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS) allow a user to precisely determine latitude, longitude, elevation, and time of day. Ranging system receivers typically accomplish this by decoding several precisely-timed signals transmitted by a group of special satellites.
For example, within the GPS system, each signal transmitted by a satellite is modulated with low frequency (typically 50 Hz) digital data which indicates the satellite's position and time of day, normalized to Greenwich Mean Time. Each satellite signal is further modulated with a unique, high frequency pseudorandom noise (PRN) code, which provide a mechanism to precisely determine the line of sight signal transmission time from each satellite.
The GPS system satellite constellation has been placed in geostationary orbit such that at least four satellites are within a direct line of sight at any given position on the earth. A typical PRN receiver thus receives a composite signal consisting of several signals transmitted by the satellites, as well as any noise and interfering signals. A decoder or channel circuit may then recover one of the transmitted signals by correlating (multiplying) the composite signal with a locally generated reference version of the PRN code signal assigned to the particular satellite of interest. If the locally generated PRN reference signal is properly timed, the digital data from that particular satellite may then be properly detected.
The signals received from different satellites are also automatically separated by the multiplying process, because the signals transmitted by different satellites use unique PRN codes having low or zero cross-correlation power. The three dimensional position of the receiver and its velocity may then be resolved by using the PRN code phase information to precisely determine the transmission time from at least four satellites, and by detecting each satellite's ephemeris and time of day data.
In order to correctly determine the offset of the PRN reference signal, its relative time delay is typically varied relative to the incoming signal until a maximum power level in the resulting correlation signal is determined. At the time offset corresponding to this point of maximum received power, the local reference signal is considered to be in synchronism with the incoming signal, and the range measurement may then be made. A so-called delay lock loop (DLL) tracking system which correlates early, punctual, and late versions of the locally generated PRN code signal against the received composite signal thus performs these operations to maintain PRN code lock in each channel.
Because of this need to precisely determine the exact propagation time, a number of problems face the designers of PRN receivers. One problem concerns accurate phase and frequency tracking of the received signals; another problem concerns the correction of relative divergence between the received signals and the local PRN code signal generators in the presence of ionospheric distortion.
In addition, because GPS systems depend upon direct line of sight for communication propagation, any multipath fading can further distort received signal timing estimates. In the ideal system, only one signal, the signal taking the direct or shortest path, is present. However, since the transmitter uses an omnidirectional (wide angle) antenna for maximum coverage, and since it is so far away from the receiver, the presence of surrounding reflecting objects such as buildings and natural surface formations means that there are typically multiple paths for the signal to take. Such a multipath signal takes a slightly different and longer router and thus arrives at the receiver at a different time.
The exact number of multipath signals present at any given moment is a function of the satellite and antenna positions relative to any and all reflecting objects. Therefore, in the typical situation, there may be none, or there may be many multipath signals. Since the multipath signals travel a longer distance they will always be received at some time after the direct path signal and will inevitably suffer a loss in power due to the reflection(s). This time delay equals the difference in length between the direct path and the reflected path divided by the propagation velocity.
The effect of the presence of multipath on the process of acquiring code lock is that there will always be some correlation with the multipath signals as well as with the desired, direct path signal.
The typical way of dealing with this is to design the PRN autocorrelation functions such that even a small offset from zero in time will yield a near zero value in the estimate of the autocorrelation function. In reality, however, the autocorrelation power decreases linearly as the time offset increases, in either the position or negative directions The multipath correlation power only reaches zero when the PRN code offset is greater than plus or minus one chip. Since the carrier sampling rate is usually much higher than the PRN code chipping rate, partial correlations will occur at sub-chip offsets.
Thus, in the presence of multipath distortion, most GPS receivers suffer a degradation in accuracy and an increase in processing time. This is especially true in high accuracy differential GPS applications, where pseudorange multipath will result in errors creeping into the differential corrections, causing large position biases.
Unlike other error sources, multipath is typically uncorrelated between antenna locations. Thus, the base and remote receivers experience different multipath interference and as a result, simple differencing between them will not cancel the errors due to multipath distortion. Also, modelling multipath for each antenna location is difficult and impractical.
A common method of reducing multipath is to carefully choose the design of the antenna and careful site selection. Unfortunately, it is often not possible to change either of these parameters. For example, if the antenna is to be mounted on an airplane fuselage, it will not be easily moved or replaced, and its shape is recessively restricted due to aerodynamic considerations.
What is needed is a way to reduce the tracking errors present in PRN ranging receivers, especially those of the lower-frequency C/A code type, in the presence of multipath fading, without degrading the signal acquisition capability of the receiver, or increasing errors due to Doppler shift, sudden receiver motion, or other noise sources. The desired method of reducing multipath distortion would be transparent to user, and operate within the GPS receiver itself, as opposed to requiring a special antenna or receiver siting.